1. Field of the Invention
The present invention relates generally to a radio frequency (RF) front-end apparatus for a Time Division Duplex (TDD) wireless communication system, and in particular, to an apparatus for suppressing an increase in an RX noise level due to the output of a high-power amplifier (HPA).
2. Description of the Related Art
In a TDD wireless communication system, an HPA is always in an ON state and a TDD scheme is implemented using a bursty RF input signal, an RF switch, or a circulator.
FIG. 1 is a block diagram of a conventional RF front-end device using an RF switch.
Referring to FIG. 1, an HPA 102 is connected to an output of a transmitter 101 and a receiver 103 is connected to an output of a low-noise amplifier (LNA) 104. In a TX mode, a single-pole double-throw (SPDT) switch 105 is switched such that a TX signal from the HPA 102 is transmitted to a filter 106. In an RX mode, the SPDT switch 105 is switched such that an RX signal from the filter 106 is transmitted to the LNA 104. The SPDT switch 105 operates to convert a TX/RX path in accordance with a TX/RX control signal. The filter 106 band-filters a TX signal and an RX signal. The conventional RF front-end device illustrated in FIG. 1 is used mainly in low-power (less than 1 W) communication systems.
FIG. 2 is a block diagram of a conventional RF front-end device using a circulator.
Referring to FIG. 2, an HPA 202 is connected to an output of a transmitter 201 and a receiver 203 is connected to an output of an LNA 204. In a TX mode, a circulator 205 operates such that a TX signal from the HPA 202 is transmitted to a filter 206. In an RX mode, the circulator 205 operates such that an RX signal from the filter 206 is transmitted to an RF switch 207. In the TX mode, the RF switch 207 is turned off to protect the LNA 204. In the RX mode, the RF switch 207 is turned on to transmit an RX signal from the circulator 205 to the LNA 204. In this way, forward signal are transmitted with little signal loss, reverse TX/RX signals are separated using the circulator 204 with high signal loss, and the RF switch 207 is used to control the input of the LNA 204 in the TX/RX modes. The conventional RF front-end device illustrated in FIG. 2 is used mainly in lower-power (less than 7˜8 W) communication systems.
In the RF front-end devices illustrated in FIGS. 1 and 2, the HPAs 102 and 202 are always in an ON state. As described above, the conventional RF front-end devices can be applied to TDD communication systems using low-power RF signals. However, in terms of the power rating and breakdown of the SPDT switch 105 and the circulator 205, the conventional RF front-end devices are problematic in communication system using high-power (more than about 10 W) RF signals. In addition, high-power SPDT switches are expensive to implement and use.
Compared to the RF front-end device illustrated in FIG. 1, the RF front-end device illustrated in FIG. 2 can be implemented at a lower price and can process a higher-power signal. However, the RF front-end device illustrated FIG. 2 is problematic in that the output level of the HPA 202 in an RX mode is much higher than a noise level and that the noise level of the receiver may increase when the high output power of the HPA 202 flows through the circulator 205 into the LNA 204. For example, when the input power level of the HPA 202 is −70 dBm in an RX mode, the output power level of the HPA 202 can be calculated according to Equation (1):−70 dBm+55 dB(HPA gain)=−15 dBm/10 MHz  (1)
The related standards provide that the output level of the HPA in an RX mode is “−96 dBm”. “−15 dBm/10 MHz” is much greater than the standard output level of the HPA. When a large leakage power is output from the HPA in an RX mode as described above, a corresponding signal-to-noise ratio (SNR) decreases. This may reduce the RX sensitivity and damage the LNA.